(1) Field of the Invention
The present invention relates to transmission devices and repeaters, and more particularly, to an optical transmission device and an optical repeater.
(2) Description of the Related Art
An increase in the transmission capacity because of recent remarkable development of the Internet has stimulated quick spreading of the optical communications network technique WDM (Wavelength Division Multiplex). WDM multiplies lights of different wavelengths and simultaneously transmits a plurality of multiplied signals over a single optical fiber.
In WDM systems, long-distance transmission has been realized in such a way that the optical signal is not converted into an electric signal at each repeater station due to the cost but is amplified by an optical amplifier.
The amplified signal from the optical amplifier is at a high level. If a fault occurs in the optical fiber transmission line or the optical transmission device, the optical signal may be emitted in the air. This situation is dangerous. For example, the dangerous situation will occur when the optical fiber cable is disconnected or pulled off from a unit. In order to prevent a maintenance person from being injured by the fault, the optical output is automatically stopped when a fault occurs.
FIG. 15 is a block diagram of an outline of a repeater station provided in a conventional WDM system. A WDM system 200 has repeater stations 210 and 220, which are coupled through optical fiber transmission lines L1 and L2. The repeater station 210 includes optical amplifiers 212a and 212b, optical couplers C1a and C1b, and a supervisory part 211. Similarly, the repeater station 220 includes optical amplifiers 222a, 222b, optical couplers C2a and C2b 
The WDM system handles not only optical main signals for services but also an optical control signal called OSC (Optical Supervisory Channel). The OSC signal includes information (DCC: Data Communication Channel) indicating the monitored states of the repeater stations and optical amplifiers and the settings of work. The optical control signal is wavelength-multiplexed into the optical main signal.
The optical main signal is amplified by the optical amplifier, while the OSC signal bypasses the optical amplifier in order to avoid interference with the optical main signal because the OSC signal is a control signal. In addition, the OSC signal is set at a comparatively low level.
The WDM system 200 operates as follows. In the up direction, the optical coupler C1a in the repeater station 210 combines the optical main signal amplified by the optical amplifier 212a and the OSC signal from the supervisory signal 211, and thus produces a multiplexed light signal. Then, the multiplexed light signal travels over the optical fiber transmission line L1 and is sent to the repeater station 220.
In the repeater station 220, the optical coupler C2a separates the optically multiplexed signal into the optical main signal and the OSC signal. The optical main signal is supplied to the optical amplifier 222a, and the OSC signal is supplied to the supervisory unit 221. In the down direction, an operation similar to that in the up direction is performed.
FIG. 16 shows a conventional light cutoff control, which is performed when a line fault occurs in the WDM system 200.
In step S100, a fault occurs in the optical fiber transmission line L1, which is disconnected.
In step S101, the supervisory part 221 of the repeater station 220 detects loss of the optical signal from the optical fiber transmission line L1 (inputting of the optical main signal and the OSC signal stops), and recognizes that a fault has occurred in the optical fiber transmission line L1.
In step S102, the supervisory part 221 stops the optical amplifier 222b amplifying the signal.
In step S103, the supervisory part 221 has fault information included in the OSC signal, and sends it to the repeater station 210 via the optical fiber transmission line L2.
In step S104, the supervisory part 211 of the repeater station 210 receives the OSC signal, and stops the optical amplifier 212a amplifying the signal.
As described above, once a fault occurs in one of the two optical fiber transmission lines, the other optical fiber transmission line that is normal is subjected to the light cutoff control (step S102).
The above control is employed taking into the account the following. In case where the optical fiber transmission line L1 is disconnected and only the optical amplifier 212a is stopped from amplifying the signal (in other words, step S102 is not performed), means for stopping the optical amplifier 222b is no longer available if the other optical fiber L2 is disconnected before the optical fiber transmission line L1 is restored.
In order to avoid such a situation, both the up and down optical fiber transmission lines are subjected to the light cutoff control even if only one of them becomes faulty. In the above-mentioned example, the OSC signal travels over the optical fiber transmission line L2. However, the OSC signal is at a comparatively low level and no dangerous situation may occur even if the optical fiber transmission line L2 is broken.
It could not be said that the system operates efficiently because the light cutoff control prevents the main signals from propagating through the normal transmission line as well as the faulty transmission line.
When the recent style of unitization of the Internet is considered, the data capacity in the down direction from the server to the user is much more than that in the up direction from the user to an ISP (Internet Service Provider), as viewed from ADSL (Asymmetric Digital Subscriber Line).
That is, the practical bidirectional transmission network has different traffics in the up and down directions. Breaking the channels having high traffic in response to disconnection of the channels having low traffic would degrade work efficiency and service quality.
As for the example of FIG. 16, it is desired to provide means for stopping the optical amplifier 222b amplifying the signal even if the optical fiber transmission line L2 is disconnected before the optical transmission line L1 is restored after it becomes faulty and the optical amplifier 212a is stopped from amplifying the signal. This would be achieved by enabling the light cutoff control with the one-way optical fiber transmission line rather than the two-way optical fiber transmission line.